1. Field of the Invention
The present invention relates to a muting circuit that temporarily interrupts an audio output. Further, the invention relates to a semiconductor integrated circuit that includes a transistor capable of muting an audio signal and is driven with a single power source.
2. Description of Related Art
Video devices and acoustic devices including a line-out jack or a headphone jack are mounted with a muting circuit that temporarily interrupts an audio output. In the muting circuit, a shot noise due to a difference in DC potential (hereinafter, referred to as a DC difference) sometimes is output when the circuit is switched from a mute-on state to a mute-off state or from a mute-off state to a mute-on state.
To reduce such a shot noise, a configuration as disclosed in Patent Document 1 (JP 9(1997)-46149 A) has been proposed. In the configuration disclosed in Patent Document 1, an output terminal of an amplifier is grounded temporarily with a bipolar transistor to mute an audio signal, thereby preventing the output of a shot noise.
Meanwhile, in recent years, as portable devices become smaller, it is becoming essential that a transistor capable of muting an audio signal is not mounted singly but is incorporated in a semiconductor integrated circuit. However, in the case where the transistor is formed of a bipolar element, the transistor could have an increased saturation voltage when it is incorporated in the semiconductor integrated circuit, depending on the manufacturing process of the semiconductor integrated circuit. When the saturation voltage of the transistor is increased, a DC difference is caused when a mute state is switched between ON and OFF, resulting in the possibility of generating a shot noise.
Hereinafter, a conventional muting circuit will be described.
FIG. 5 shows a conventional muting circuit 200. In FIG. 5, a muting transistor unit 100 includes a first transistor 101 and a second transistor 102 that are npn transistors. In the first transistor 101, an emitter is connected to a mute terminal 120, a collector is connected to a collector of the second transistor 102, and a base is connected to a resistor 111. In the second transistor 102, the collector is connected to the collector of the first transistor 101, a base is connected to a resistor 112, and an emitter is grounded.
In a third transistor 103 that is a pnp transistor, an emitter is connected to a power source Vcc, a base is connected to a collector of a fifth transistor 105, and a collector is connected to a base of a fourth transistor 104.
In the fourth transistor 104 that is a npn transistor, a collector is connected to the power source Vcc, the base is connected to the collector of the third transistor 103, and an emitter is connected to the base of the first transistor 101 via the resistor 111 as well as to the base of the second transistor 102 via the resistor 112.
The fifth transistor 105 and a sixth transistor 106 that are pnp transistors constitute a current mirror, in which respective emitters are connected to the power source Vcc. The collector of the fifth transistor 105 is connected to the base of the third transistor 103 and is grounded via a resistor 114. A collector of the sixth transistor 106 is connected to a current source 116 via a switch 115.
An amplifier 117 amplifies an audio signal output from an audio signal processing circuit (not shown) and outputs the same. The amplified audio signal output from the amplifier 117 is output to the outside from an audio output terminal 121 of the semiconductor integrated circuit 200.
Hereinafter, an operation will be described.
In the muting circuit shown in FIG. 5, in a mute-off state, the switch 115 is ON, so that a current Imute flows through the current source 116 and also through the resistor 114. When the current Imute flows through the resistor 114, the third transistor 103 has an increased base voltage, and is turned OFF. Accordingly, the current is not supplied to the base of the fourth transistor 104, so that the fourth transistor 104 also is turned OFF. Because the fourth transistor 104 is turned OFF, the current is not supplied to the bases of the first transistor 101 and the second transistor 102, so that the first transistor 101 and the second transistor 102 are turned OFF. Consequently, the mute terminal 120 has a high impedance, and has no effect on an audio signal output from the audio output terminal 121. Thus, the audio signal output from the audio output terminal 121 is output to a load side.
On the other hand, in a mute-on state, the switch 115 is OFF as shown in FIG. 5, so that the current Imute does not flow. Accordingly, the base voltage of the third transistor 103 is pulled down by the resistor 114, so that the third transistor 103 is turned ON. Because the third transistor 103 is turned ON, the current is supplied to the base of the fourth transistor 104, so that the fourth transistor 104 is turned ON. Because the fourth transistor 104 is turned ON, the current is supplied to the bases of the first transistor 101 and the second transistor 102, so that the first transistor 101 and the second transistor 102 are turned ON. Because the first transistor 101 and the second transistor 102 are turned ON, a capacitor 118 one end of which is connected to the audio output terminal 121 is grounded at the other end. Consequently, the audio signal output from the audio output terminal 121 is not output to the load side.
However, since the first transistor 101 and the second transistor 102 are bipolar transistors, a shot noise could be generated.
More specifically, the bipolar transistors could have an increased saturation voltage. When the saturation voltage is increased, a DC difference is caused when the mute state is switched between ON and OFF, resulting in a shot noise.
FIG. 6 shows a signal waveform when the mute state is switched in the conventional muting circuit. In FIG. 6, (a) indicates the ON/OFF state of the switch 15, (b) indicates a voltage of the mute terminal 120, and (c) indicates the output audio signal, in an exemplary case where no audio is output. As shown in FIG. 6, when the switch 15 is turned OFF so that the mute state is switched from ON to OFF, the saturation voltage of the muting transistor 100 causes a DC difference in the mute terminal 120 (see (b) in FIG. 6), which results in the generation of a shot noise (see (c) in FIG. 6). The shot noise thus generated is output at an audible volume, making a listener uncomfortable.